depending on your level . . .
I just "power leveled" my mining as a 60 DK.
This is the Mithril path i took:
Goto Stone Talon Mts and run around the crater (charred valley). Then zone down into Desolace, and run around the zone, ending back at the top of the zone. Check the charred valley and repeat.
I also ran around W. Plague Lands, mith is in there too.
And if both zones had people I went to Hinterlands
If you want real easy mode, down load Gatherer add-on, and then get the WoWHead Database add-on for Gatherer, and Cartographer.
Once all is set up and you download the database, you will be shown ALL mining nodes on the map. Filter out one you don't really need to see and chart a course to run.
I would suggest W. Plague Lands when it is time to move onto the next ore (thorium ? i can't spell..) as most in the one will be Small. Get the easy hits, and when you get closer to 285 (the skill needed for Rich iirc) then move to Un'goro Crater (crater is usually packed with people, and the last 15 points can suck if there are so many there).
Now if you waited out to lvl a skill, and you are like 70+ when you get to thorium, you could goto sithillius (sp). You will probably have the entire zone to yourself as most stay in the Crater